1. Field of the Invention
The invention relates to communications receivers, and particularly, to apparatus for use in such a receiver for automatically detecting a constellation size of a received quadrature amplitude modulated (QAM) signal.
2. Description of the Prior Art
Various communications systems utilize quadrature amplitude modulation (QAM) for transmission of relatively high data rate information within a limited transmission bandwidth. Typically, QAM communications systems use a fixed symbol constellation for all transmissions, e.g., sixteen positions or points within a constellation. Conventional QAM receivers are capable of receiving transmissions of only a single symbol constellation size.
However, recently, sophisticated QAM communications systems are capable of transmitting variable symbol rate transmissions using two or more symbol constellations. For example, such a QAM system could vary its symbol rate between a 32-ary constellation and a 16-ary constellation depending upon the presence of atmospheric noise. Specifically, during periods of low atmospheric noise, such a QAM system can use 32-ary transmissions. When atmospheric noise, as measured at the transmitter, has increased above a pre-established noise threshold, the constellation size is then decreased to a 16-ary constellation. During periods of extremely low noise, the constellation size could be increased to transmissions of 32-ary, 64-ary or even 128-ary constellations. Alternatively, a QAM system may be required to transmit one constellation size over-the-air and a second constellation size over a cable broadcast system. As such, a "cable ready" receiver must be able to receive both constellation sizes.
One such variable constellation size communication system is being considered by the Federal Communications Commission (FCC) as a standard transmission format for high definition television (HDTV). The particular standard would permit HDTV broadcasters to use either 16-ary or 32-ary QAM symbol constellations for broadcasting HDTV signals. As such, a given HDTV receiver may receive a 16-ary transmission when viewing one particular channel and a 32-ary transmission when viewing a second channel. Additionally, a given HDTV broadcaster may change its transmitted symbol constellation from 16- to 32-ary, or vice versa, whenever noise conditions permit the higher (lower) rate of transmission to a majority of the broadcast audience. Thus, an HDTV receiver must be capable of automatically determining whether a received broadcast is a 16- or 32-ary transmission. Such a determination must be accomplished whenever a user changes channels or the broadcaster changes transmission rates. Additionally, the constellation size determination must be accomplished relatively quickly such that a user will not notice the change in the constellation size of the broadcast, i.e., notice a loss of signal reception while the receiver adjusts to a new constellation size.
Furthermore, HDTV cable broadcasts may utilize 64-ary transmissions. As such, a "cable ready" HDTV receiver must be capable of receiving 16, 32 and 64-ary transmissions.
Typically, a receiver of variable constellation size transmissions contains demodulator circuits capable of demodulating each expected size of symbol constellation. In particular, a receiver capable of receiving both 16- and 32-ary symbol constellations would contain both a 16-ary demodulator and a 32-ary demodulator. In this manner, both symbol constellations are demodulated simultaneously even though only one constellation is transmitted. Thus, one demodulator produces no signal, while the other demodulator demodulates the received symbols and generates information therefrom. Though such a receiver accomplishes reception of either symbol constellation, such redundant demodulators are complex and costly to manufacture.
Using an alternative technique to determine constellation size, an HDTV receiver contains a single demodulator having two modes of operation, i.e., one for each expected symbol constellation size. As such, the demodulator attempts to demodulate one of the constellation sizes, e.g., a 16-ary symbol constellation, using one of its two modes of operation. Meanwhile, circuitry within the receiver monitors an error rate from a Reed-Solomon decoder within the demodulator. If the error rate exceeds a pre-defined threshold, the receiver assumes that the demodulator is set to demodulate the incorrect symbol constellation size. In response, the receiver switches the demodulator to a second mode of operation, e.g., a 32-ary symbol constellation mode, capable of demodulating a second symbol constellation size, e.g., 32-ary. Simply stated, the error rate at the output of the Reed-Solomon decoder indicates which constellation is presently being demodulated. However, for a Reed-Solomon decoder to operate properly, a carrier recovery circuit must be locked onto a carrier for the transmission. Otherwise, the Reed-Solomon decoder produces a high error rate for both modes of operation. As such, this form of receiver first requires that a carrier lock be achieved before determining the constellation size. Detrimentally, this two-step process is relatively slow to determine the constellation size of the received broadcast. Consequently, a large amount of information can be lost while the receiver is achieving carrier lock and then determining the constellation size before beginning to demodulate the broadcast.
Therefore, a need exists in the art for apparatus, particularly though not exclusively for use in a HDTV QAM receiver, for automatically detecting the constellation size of a QAM transmission without requiring the QAM demodulator to, a priori, acquire carrier lock. Additionally, to minimize any noticeable signal impact to a viewer, this detection should be performed relatively quickly.